With the white/grey transmog getting enabled they also added the vanilla starting gear to the Stormwind/Orgrimmar armor vendors. There are still a couple of missing ones though.
First one that comes to mind is the human warrior starting gear. This one seems especially weird in that it flat out seems to have been removed from the game entirely. Unlike the other gear which was still there for people who still owned the items in their inventory/bank after they were no longer obtainable. The one you buy from the Stormwind vendor is the post-Cata warrior gear which is a generic brown vest, while the vanilla one was a white shirt with leather belt straps. You can only find it in the WoWHead Classic databse.
The second that comes to mind is the troll shaman starting gear which was distinct from the other horde shamans. I’m sure there’s possibly even more that aren’t accounted for.